1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) device.
2. Description of the Related Art
An organic EL device receives attention because it can be driven with low power and is capable of achieving high intensity light emission. Thus, research and development concerning the organic EL device is being actively conducted. Generally, the organic EL device has a structure in which a fluorescent layer formed of an organic material is sandwiched between a pair of electrodes, at least one of which has optical transparency. A hole injection layer, a hole transport layer, an electron injection layer, an electron transport layer and the like between a pair of electrodes are provided.
The organic EL device can be used in, for example, a display apparatus. Drive systems of the display apparatus include a simple matrix system and an active matrix system, but the display apparatus of the simple matrix system has a problem in that power consumption increases as a display panel is increased in size because the instantaneous luminance of each pixel should be enhanced as the duty ratio increases. Thus, if a larger display panel is needed, the active matrix system is mainly used.
The display apparatus of the active matrix system using the organic EL device has a substrate on which a plurality of thin film transistors (hereinafter abbreviated as “TFT”) are formed, and an organic EL device formed on the substrate. The TFT is formed for each of the pixels arranged in a matrix form. By turning the TFT of each pixel ON/OFF with a control signal, the light emission state of the organic EL device can be controlled for each pixel and as a result, an image can be displayed. Display apparatuses having such a configuration have generally used a system (bottom emission system) in which light from the organic EL device is extracted from the side opposite to the side of the TFT substrate on which the TFT is formed.
The organic EL device is classified into a low-molecular type and a polymer type depending on the type of a light emission material forming a fluorescent layer. The low-molecular type organic EL device has a fluorescent layer made of low-molecular light emission material, and a polymer type organic EL device has a fluorescent layer made of polymer light emission material.
In both the low-molecular type and the polymer type organic EL devices of the bottom emission system, ITO (indium tin oxide) and IZO (indium zinc oxide) having relatively large work functions are used as anodes. As preferred cathodes, for example, an alloy single layer film of Mg (magnesium) and silver (Ag), a layered film of a halogenated alkali metal such as LiF and Al, and the like are used for the low-molecular type organic EL device and for example, a layered film of a low-work function metal such as Ca or Ba and a relatively stable protective metal layer of Al, Ag or the like is used for the polymer type organic EL device. In this way, the low-molecular type organic EL device and the polymer type organic EL device are different in the preferred cathode material used. This is because the LUMO level of the polymer light emission material is lower than the LUMO level of the low-molecular light emission material, and therefore, the polymer type organic EL device preferably uses a cathode material having a smaller work function for effective injection of electrons.
If an active matrix type full color display is fabricated using the organic EL device, a pattern of an RGB (red, green and blue) fluorescent layer should be formed using mask vapor deposition for the low-molecular type organic EL device. Accordingly, enhancement of fineness (e.g. 200 PPI, etc.) of the organic EL device is difficult, and it is difficult to form the organic EL device on a glass substrate having a large area (e.g., about 1 m square), resulting in a problem that efficient production steps using such a glass substrate cannot be used. The polymer type organic EL device can be fabricated by a wet process, and therefore requires no mask vapor deposition or the like. Further, an organic EL device having high fineness can be formed on a glass substrate having a large area by a method such as inkjet printing. Thus, it is possible that the polymer type EL device is more advantageous than the low-molecular type EL device in terms of productivity and production costs.
If the active matrix type display apparatus using the organic EL device is fabricated, use of the bottom emission system results in the following problems.
The TFT usually has a semiconductor layer made of amorphous silicon or the like, and an electrode made of metal, but the semiconductor layer and electrode do not have sufficient optical transparency. Accordingly, the bottom emission system in which light is extracted through the TFT substrate has a problem in that the ratio of the pixel area to the light emission area (aperture ratio) decreases. For the organic EL display apparatus, a current drive system is suitably used because variations in display performance for each pixel are reduced, and a change in panel display luminance due to degradation of the organic EL material can be reduced, but 4 transistors are required for each pixel if the current drive system is used. Accordingly, the aperture ratio further decreases compared with the use of a voltage drive system (requiring 2 transistors for each pixel) that is simpler but inferior in variations in display for each pixel, and the like (Shang-Li Chen et al. IDW '01, p. 399).
Thus, a display apparatus configuration using a system (top emission system) in which light from the organic EL device is extracted above the TFT and organic EL device formed on the substrate has been proposed. In the top emission system, the problem with the aperture ratio can be prevented. However, in this system, firstly, the cathode, which is the upper electrode, of the organic EL device should be transparent so as to have light emission. Secondly, the cathode should be made of a material having a small work function for efficiently injecting electrons into the fluorescent layer. Thirdly, as described in detail below, damage to the lower layer resulting from processes of fabrication of the cathode should be prevented where possible. The cathode usually has a layered structure of an electron injecting electrode made of a very thin metal film so that light can be transmitted, and a transparent conductive film formed on the electron injecting electrode. The transparent conductive film is provided for protecting the electron injecting electrode being a thin metal layer and reducing a wiring resistance. For formation of the transparent conductive film, a method of generating particles of relatively high energy, such as sputtering or ion plating, is usually used, and therefore the electron injecting electrode, fluorescent layer and the like provided in the lower layer may be damaged, resulting in degradation in device characteristics.
Various configurations have been proposed for this cathode in top emission system devices. Those conventional configurations are principally based on the concept that a cathode material and an electron injection material capable of realizing favorable characteristics in the bottom emission system are transferred and applied as directly as possible to the cathode of the top emission system, and the thickness of an electrode layer made of those materials is reduced so that light emission from the cathode of the top emission system is realized.
For example, Japanese Laid-Open Patent Publication No. 2001-85163 discloses that an alloy of Mg and Ag being one of the cathode materials in the organic EL device of the bottom emission system (Japanese Patent No. 2814435, etc.) is used for the upper electrode or cathode of the top emission system. The low-molecular type EL device in this document includes a metal layer (Mg—Ag layer) having a sufficiently small thickness (10 nm) and a transparent conductive layer (IZO layer) formed in this order on a low-molecular fluorescent layer. Furthermore, Japanese Laid-Open Patent Publication No. 10-162959 discloses a low-molecular type organic EL device of the top emission system including a cathode having a layered structure of an electron injection layer and an amorphous transparent conductive film.
However, the above-described documents both deal with the configuration of the cathode where the low-molecular type organic EL device is used. As described above, the polymer type organic EL device and the low-molecular type organic EL device have different electrode configurations and electrode materials providing satisfactory characteristics, and therefore it is necessary to consider the configuration of the cathode that is useful for the polymer type organic EL device independently.
Furthermore, the electrode configuration in the above document also has the following problem.
A conductive oxide layer containing indium tin oxide (ITO) or indium zinc oxide (IZO) as a main component is used as a transparent conductive film in terms of optical transparency and resistivity, but the satisfactory conductive oxide layer made of ITO or IZO is generally formed while introducing oxygen gas into a film forming apparatus. The cathode has a metal layer for injection of electrons into the organic EL device, and for the material of this metal layer, a metal having a small work function (hereinafter referred to as “low work function metal”), for example, an alkali earth metal such as magnesium (Mg), calcium (Ca) or Barium (Ba), an alkali metal, indium (In) or an alloy of any of these metals is often used for improving the electron injection efficiency. Accordingly, if a metal layer made of low work function metal is formed, and a conductive oxide layer is formed with this metal layer as a ground, the low work function metal is easily oxidized with oxygen gas introduced into the film forming apparatus, resulting in degradation in device characteristics. Furthermore, even if the conductive oxide layer is formed while introducing only Ar gas into the film forming apparatus without introduction of oxygen gas, device characteristics may be degraded because the low work function metal is oxidized by oxygen contained in the conductive oxide layer.
Thus, there is the problem of oxidation of the metal layer particularly when a conductive oxide layer of ITO or the like is formed on a metal layer made of low work function metal, but the problem of degradation in characteristics or reliability associated with oxidation of the metal layer arises as long as the metal layer contacts the conductive oxide layer, irrespective of the order of formation. This problem with oxidation of the metal layer is especially serious in the case of the polymer type organic EL device using a metal having a smaller work function.